In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engage able friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. Each such shift includes a fill or preparation phase during which an apply chamber of the on-coming clutch is filled in preparation for torque transmission. Once filled, the on-coming clutch transmits torque in relation to the clutch pressure, and the shift can be completed using various control strategies.
Typically, the fill phase is carried out by commanding a given on-coming clutch fill pressure for an empirically determined fill time. See, for example, the U.S. Pat. No. 4,707,789 to Downs et al., issued on Nov. 17, 1987, and assigned to the assignee of the present invention. In Downs et al., the fill time for any given clutch is obtained from a look-up table as a function of the commanded fill pressure, less the clutch return spring pressure, and then adjusted as a function of fluid temperature to compensate for variations in fluid viscosity. Another approach is found in the U.S. Pat. No. 5,216,606 to Lentz et al., issued on Jun. 1, 1993, and assigned to the assignee of the present invention. In Lentz et al., where the fill time is determined by table look-up based on the pump speed, as compensated for efficiency and fluid viscosity, and further modified based on the time since the last shift and a hydraulic restriction factor.
It has additionally been recognized that control strategies of the above-described type cannot effectively account for vehicle-to-vehicle manufacturing variations or for variations that occur within a given vehicle over time due to wear, changing fluid characteristics, etc. For this reason, various control methods have been developed for adaptively adjusting the stored table look-up values in order to compensate for sources of error that influence the fill time. In the aforementioned U.S. Patent to Downs et al., for example, the inertia phase delay of the shift is monitored, and used to form an adaptive fill time correction for subsequent similar shifts if the monitored delay significantly deviates from a desired value. Another approach is described in the U.S. Pat. No. 5,072,390 to Lentz et al., issued on Dec. 10, 1991, and also assigned to the assignee of the present invention. There, the input speed is monitored to detect an aberrant event such as flaring, early pull-down or sharp deceleration, and the stored fill time for shifts of that type is adaptively adjusted based on the time difference between expected end of fill and the detection of the aberrant event.
A drawback of the above-described control methods is that they depend extensively on empirically determined data, and the shift parameter (fill time) represented by the stored table values is influenced by numerous unmeasured variables. As a result, the source of a detected fill time error usually cannot be determined, and it is difficult to know how much authority or gain should be accorded to the adaptive adjustment.